The invention relates to the separation of gases from hydrocarbon gas mixtures. In particular, the invention relates to the separation of hydrogen from hydrocarbons. The separation is carried out using hydrocarbon-resistant membranes, and is useful in refineries, petrochemical plants, and the like.
Polymeric gas-separation membranes are well known and are in use in such areas as production of oxygen-enriched air, production of nitrogen from air, separation of carbon dioxide from methane, hydrogen recovery from various gas mixtures and removal of organic vapors from air or nitrogen.
The preferred membrane for use in any gas-separation application combines high selectivity with high flux. Thus, the membrane-making industry has engaged in an ongoing quest for polymers and membranes with improved selectivity/flux performance. Many polymeric materials are known that offer intrinsically attractive properties. That is, when the permeation performance of a small film of the material is measured under laboratory conditions, using pure gas samples and operating at modest temperature and pressure conditions, the film exhibits high permeability for some pure gases and low permeability for others, suggesting useful separation capability.
Unfortunately, gas separation in an industrial plant is seldom so simple. The gas mixtures to which the separation membranes are exposed may be hot, contaminated with solid or liquid particles, or at high pressure, may fluctuate in composition or flow rate or, more likely, may exhibit several of these features. Even in the most straightforward situation possible, where the gas stream to be separated is a two-component mix, uncontaminated by other components, at ambient temperature and moderate pressure, one component may interact with the membrane in such a way as to change the permeation characteristics of the other component, so that the separation factor or selectivity suggested by the pure gas measurements cannot be achieved. In gas mixtures that contain condensable components, it is frequently, although not always, the case that the mixed gas selectivity is lower, and at times considerably lower, than the ideal selectivity. The condensable component, which is readily sorbed into the polymer matrix, swells or, in the case of a glassy polymer, plasticizes the membrane, thereby reducing its selective capabilities. A technique for predicting mixed gas performance under real conditions from pure gas measurements with any reliability has not yet been developed.
A good example of these performance problems is the separation of hydrogen from mixtures containing hydrogen, methane and other hydrocarbons. Increasing reliance on low-hydrogen, high-sulfur crudes, coupled with tighter environmental regulations, has raised hydrogen demand in refineries. This is primarily due to increased hydrodesulfurization and hydrocracking; as a result many refineries are now out of balance with respect to hydrogen supply. At the same time, large quantities of hydrogen-containing off-gas from refinery processes are currently rejected to the refinery""s fuel gas systems. Besides being a potential source of hydrogen, these off-gases contain hydrocarbons of value, for example, as liquefied petroleum gas (LPG) and chemical feedstocks.
The principal technologies available to recover hydrogen from these off-gases are cryogenic separation, pressure swing adsorption (PSA), and membrane separation. Membrane gas separation, the newest, is based on the difference in permeation rates of gas components through a selective membrane. Many membrane materials are much more permeable to hydrogen than to other gases and vapors. One of the first applications of gas separation membranes was recovery of hydrogen from ammonia plant purge streams, which contain hydrogen and nitrogen. This is an ideal application for membrane technology, because the membrane selectivity is high, and the feed gas is clean (free of contaminants, such as heavier hydrocarbons). Another successful application is to adjust hydrogen/carbon monoxide or hydrogen/methane ratios for synthesis gas production. Again, the feed gas is free of heavy hydrocarbon compounds.
Application of membranes to refinery separation operations has been much less successful. Refinery gas streams contain contaminants such as water vapor, acid gases, olefins, aromatics, and other organics. At relatively low concentrations, these contaminants cause membrane plasticization and loss of selectivity. At higher concentrations they can condense on the membrane and cause irreversible damage to it. When a feedstream containing such components and hydrogen is introduced into a membrane system, the hydrogen is removed from the feed gas into the permeate and the gas remaining on the feed side becomes progressively enriched in hydrocarbons, raising the dewpoint. For example, if the total hydrocarbon content increases from 60% in the feed gas to 85% in the residue gas, the dewpoint may increase by as much as 25xc2x0 C. or more, depending on the hydrocarbon mix. Maintaining this hydrocarbon-rich mixture as gas may require it to be maintained at high temperature, such as 60xc2x0 C., 70xc2x0 C., 80xc2x0 C. or even higher, which is costly and may itself eventually adversely affect the mechanical integrity of the membrane. Failure to do this means the hydrocarbon stream may enter the liquid-phase region of the phase diagram before it leaves the membrane module, and condense on the membrane surface, damaging it beyond recovery. Even if the hydrocarbons are kept in the gas phase, separation performance may fall away completely in the presence of hydrocarbon-rich mixtures.
These issues are discussed, for example, in J. M. S. Henis, xe2x80x9cCommercial and Practical Aspects of Gas Separation Membranesxe2x80x9d Chapter 10 of D. R. Paul and Y. P. Yampol""skii, Polymeric Gas Separation Membranes, CRC Press, Boca Raton, 1994. This reference gives upper limits on various contaminants in streams to be treated by polysulfone membranes of 50 psi hydrogen sulfide, 5 psi ammonia, 10% saturation of aromatics, 25% saturation of olefins and 11xc2x0 C. above paraffin dewpoint (pages 473-474).
A great deal of research has been performed on improved membrane materials for hydrogen separation. A number of these materials appear to have significantly better properties than the original cellulose acetate or polysulfone membranes. For example, modern polyimide membranes have been reported with selectivity for hydrogen over methane of 50 to 200, as in U.S. Pat. Nos. 4,880,442 and 5,141,642. Unfortunately, these materials appear to remain susceptible to severe loss of performance through plasticization and to catastrophic collapse if contacted by liquid hydrocarbons. Several failures have been reported in refinery applications where these conditions occur. This low process reliability has caused a number of process operators to discontinue applications of membrane separation for hydrogen recovery.
Thus, the need remains for membranes that will provide and maintain adequate performance under the conditions of exposure to gas mixtures, and particularly those containing C3+ hydrocarbons, that are commonplace in refineries, chemical plants, or gas fields.
Films or membranes made from fluorinated polymers having a ring structure in the repeat unit are known. For example:
1. U.S. Pat. Nos. 4,897,457 and 4,910,276, both to Asahi Glass, disclose various perfluorinated polymers having repeating units of perfluorinated cyclic ethers, and cite the gas-permeation properties of certain of these, as in column 8, lines 48-60 of 4,910,276.
2. A paper entitled xe2x80x9cA study on perfluoropolymer purification and its application to membrane formationxe2x80x9d (V. Arcella et al., Journal of Membrane Science, Vol. 163, pages 203-209 (1999)) discusses the properties of membranes made from a copolymer of tetrafluoroethylene and a dioxole. Gas permeation data for various gases are cited.
3. European Patent Application 0 649 676 A1, to L""Air Liquide, discloses post-treatment of gas separation membranes by applying a layer of fluoropolymer, such as a perfluorinated dioxole, to seal holes or other defects in the membrane surface.
4. U.S. Pat. No. 5,051,114, to Du Pont, discloses gas separation methods using perfluoro-2,2-dimethyl-1,3-dioxole polymer membranes. This patent also discloses comparative data for membranes made from perfluoro(2-methylene-4-methyl-1,3-dioxolane) polymer (Example XI).
5. A paper entitled xe2x80x9cGas and vapor transport properties of amorphous perfluorinated copolymer membranes based on 2,2-bistrifluoromethyl-4,5-difluoro-1,3-dioxole/tetrafluoroethylenexe2x80x9d (I. Pinnau et al., Journal of Membrane Science, Vol. 109, pages 125-133 (1996)) discusses the free volume and gas permeation properties of fluorinated dioxole/tetrafluoroethylene copolymers compared with substituted acetylene polymers. This reference also shows the susceptibility of this dioxole polymer to plasticization by organic vapors and the loss of selectivity as vapor partial pressure in a gas mixture increases (FIGS. 3 and 4).
Most of the data reported in the prior art references listed above are for permanent gases, carbon dioxide and methane, and refer only to measurements made with pure gases. The data reported in item 5 indicate that even these fluorinated polymers, which are characterized by their chemical inertness, appear to be similar to conventional hydrogen-separating membranes in their inability to withstand exposure to propane and heavier hydrocarbons.
The invention is a process for separating hydrogen from a gaseous hydrocarbon in a gas mixture. Such a mixture might typically, but not necessarily, be found as a process or waste stream from a petrochemical plant or a refinery. The mixture is typically a multicomponent mixture, containing the gaseous hydrocarbon from which it is desired to separate hydrogen, as well as at least one other gaseous hydrocarbon, and frequently containing other components such as nitrogen, carbon dioxide or water vapor, for example.
The separation is carried out by running a stream of the gas mixture across a membrane that is selective for hydrogen over the hydrocarbon from which it is to be separated. The process results, therefore, in a permeate stream enriched in hydrogen gas and a residue stream depleted in hydrogen gas. The process can separate hydrogen from methane, hydrogen from ethylene, hydrogen from ethane, hydrogen from C3+ hydrocarbon vapors, hydrogen from halogenated hydrocarbons, or any combination of these, for example.
The process differs from processes previously available in the art in that:
(i) the membranes are able to provide useful separation properties for multicomponent gas mixtures, including, but not limited to, gas mixtures containing C3+ hydrocarbon vapors and/or carbon dioxide, even at high levels in the gas mixture, and
(ii) the membranes can recover from accidental exposure to liquid organic compounds.
To provide these attributes, the membranes used in the process of the invention are made from a glassy polymer or copolymer. The polymer is characterized by having repeating units of a fluorinated, cyclic structure, the ring having at least five members. The polymer is further characterized by a fractional free volume no greater than about 0.3 and preferably by a glass transition temperature, Tg, of at least about 100xc2x0 C. Preferably, the polymer is perfluorinated.
In the alternative, the membranes are characterized in terms of their selectivity before and after exposure to liquid hydrocarbons. Specifically, the membranes have a post-exposure selectivity for hydrogen over the gaseous hydrocarbon from which it is desired to separate hydrogen, after exposure of the separation membrane to a liquid hydrocarbon, for example, toluene, and subsequent drying, that is at least about 60%, 65% or even 70% of a pre-exposure selectivity for hydrogen over the gaseous hydrocarbon, the pre- and post-exposure selectivities being measured with a test gas mixture of the same composition and under like conditions.
In this case, the selective layer is again made from an amorphous glassy polymer or copolymer with a fractional free volume no greater than about 0.3 and a glass transition temperature, Tg, of at least about 100xc2x0 C. The polymer is fluorinated, generally heavily fluorinated, by which we mean having a fluorine:carbon ratio of atoms in the polymer of at least about 1:1. Preferably, the polymer is perfluorinated. In this case the polymer need not incorporate a cyclic structure.
In a basic embodiment, the process of the invention includes the following steps:
(a) bringing a multicomponent gas mixture comprising hydrogen, a gaseous hydrocarbon, and a third gaseous component into contact with the feed side of a separation membrane having a feed side and a permeate side, the membrane having a selective layer comprising: a polymer comprising repeating units having a fluorinated cyclic structure of an at least 5-member ring, the polymer having a fractional free volume no greater than about 0.3;
(b) providing a driving force for transmembrane permeation;
(c) withdrawing from the permeate side a permeate stream enriched in hydrogen compared to the gas mixture;
(d) withdrawing from the feed side a residue stream depleted in hydrogen compared to the gas mixture.
In the alternative, a basic embodiment of the process includes the following steps:
(a) bringing a multicomponent gas mixture comprising hydrogen, a gaseous hydrocarbon, and a third gaseous component into contact with the feed side of a separation membrane having a feed side and a permeate side, the membrane having a selective layer comprising a polymer having:
(i) a ratio of fluorine to carbon atoms in the polymer greater than 1:1;
(ii) a fractional free volume no greater than about 0.3; and
(iii) a glass transition temperature of at least about 100xc2x0 C.; and the separation membrane being characterized by a post-exposure selectivity for hydrogen over the first gaseous hydrocarbon, after exposure of the separation membrane to liquid toluene and subsequent drying, that is at least about 65% of a pre-exposure selectivity for hydrogen over the gaseous hydrocarbon, as measured pre- and post-exposure with a test gas mixture of the same composition and under like conditions;
(b) providing a driving force for transmembrane permeation;
(c) withdrawing from the permeate side a permeate stream enriched in hydrogen compared to the gas mixture;
(d) withdrawing from the feed side a residue stream depleted in hydrogen compared to the gas mixture.
The permeate stream or the residue stream, or both, may be the useful products of the process.
Examples of hydrocarbons from which hydrogen may be separated include, but are not limited to, paraffins, both straight and branched, for example, methane, ethane, propane, butanes, pentanes, hexanes; olefins and other aliphatic unsaturated organics, for example, ethylene, propylene, butene; aromatic hydrocarbons, for example, benzene, toluene, xylenes; vapors of halogenated solvents, for example, methylene chloride, perchloroethylene; alcohols; ketones; and diverse other volatile organic compounds. In many cases, the gas mixture to be treated contains multiple of these components.
Particularly preferred materials for the selective layer of the membrane used to carry out the process of the invention are amorphous homopolymers of perfluorinated dioxoles, dioxolanes or cyclic alkyl ethers, or copolymers of these with tetrafluoroethylene. Specific most preferred materials are copolymers having the structure: 
where x and y represent the relative proportions of the dioxole and the tetrafluoroethylene blocks, such that x+y=1.
A second highly preferred material has the structure: 
where n is a positive integer.
Contrary to what would be expected from the data presented in the Pinnau et al. Journal of Membrane Science paper, we have unexpectedly found that membranes formed from fluorinated cyclic polymers as characterized above can withstand exposure to C3+ hydrocarbons well enough to provide useful separation capability for gas mixtures that include C3+ hydrocarbon vapors. This resistance persists even when the C3+ hydrocarbons are present at high levels, such as 5%, 10%, 15% or even more.
Thus, a particularly important advantage of the invention is that the membranes can retain selectivity for hydrogen even in the presence of streams rich in, or even essentially saturated with, C3+ hydrocarbon vapors. This distinguishes these membrane materials from all other membrane materials previously used commercially for hydrogen separations.
Membranes made from fluorinated dioxoles have been believed previously to behave like conventional membrane materials in suffering from debilitating plasticization in a hydrocarbon containing environment, to the point that they may even become selective for hydrocarbons over permanent gas even at moderate C3+ hydrocarbon partial pressures. We have discovered that this is not the case for the membranes taught herein. This unexpected result is achieved because the membranes used in the invention are unusually resistant to plasticization by hydrocarbon vapors.
The membranes are also resistant to contact with liquid hydrocarbons, in that they are able to retain their selectivity for hydrogen over methane after prolonged exposure to liquid toluene, for example. This is a second beneficial characteristic that differentiates the processes of the invention from prior art processes. In the past, exposure of the membranes to liquid hydrocarbons frequently meant that the membranes were irreversibly damaged and had to be removed and replaced.
Besides withstanding exposure during use, their resistance to hydrocarbons enables the membranes and modules to be cleaned with hydrocarbon solvents to remove oils or other organic materials that may have been deposited during operation. This is an additional and beneficial improvement over processes previously available in the art.
These unexpected and unusual attributes render the process of the invention useful, not only in situations where commercial gas separation membranes have been used previously, but also in situations where it was formerly difficult or impractical for membrane separation to be used, or where membrane lifetimes were poor.
Because the preferred polymers are glassy and rigid, an unsupported film of the polymer may be usable in principle as a single-layer gas separation membrane. However, such layer will normally be far too thick to yield acceptable transmembrane flux, and in practice, the separation membrane usually comprises a very thin selective layer that forms part of a thicker structure, such as an asymmetric membrane or a composite membrane. The making of these types of membranes is well known in the art.
If the membrane is a composite membrane, the support layer may optionally be made from a fluorinated polymer also, making the membrane a totally fluorinated structure and enhancing chemical resistance. The membrane may take any form, such as hollow fiber, which may be potted in cylindrical bundles, or flat sheets, which may be mounted in plate-and-frame modules or formed into spiral-wound modules.
The driving force forpermeation across the membrane is the pressure difference between the feed and permeate sides, which can be generated in a variety of ways. The pressure difference may be provided by compressing the feedstream, drawing a vacuum on the permeate side, or a combination of both. The membrane is able to tolerate high feed pressures, such as above 200 psia, 300 psia, 400 psia or more. As mentioned above, the membrane is able to operate satisfactorily in the presence of C3+ hydrocarbons at high levels. Thus the partial pressure of the hydrocarbons in the feed may be close to saturation. For example, depending on the mix of hydrocarbons and the temperature of the gas, the aggregate partial pressure of all C3+ hydrocarbons in the gas might be as much as 10 psia, 15 psia, 25 psia, 50 psia, 100 psia, 200 psia or more. Expressed as a percentage of the saturation vapor pressure at that temperature, the partial pressure of hydrocarbons, particularly C3+ hydrocarbons, may be 20%, 30%, 50% or even 70% or more of saturation.
The membrane separation process may be configured in many possible ways, and may include a single membrane unit or an array of two or more units in series or cascade arrangements. The processes of the invention also include combinations of the membrane separation process defined above with other separation processes, such as adsorption, absorption, distillation, condensation or other types of membrane separation.
The scope of the invention in this aspect is not intended to be limited to any particular gas streams, but to encompass any situation where a multicomponent gas stream containing at least hydrogen, a hydrocarbon gas and a third component is to be separated. The composition of the gas may vary widely, from a mixture that contains minor amounts of hydrogen in admixture with various hydrocarbon components, including relatively heavy hydrocarbons, such as C5-C8 hydrocarbons or heavier, to a mixture of mostly hydrogen, such as 80% hydrogen, 90% hydrogen or above, with methane and other very light components, to an essentially binary mixture of hydrogen and methane with only very small amounts of other minor components, such as carbon dioxide or water vapor.
The process of the invention typically provides a selectivity, as measured with the gas mixture to be separated, even if the gas contains significant amounts of C3+ hydrocarbon vapor, for hydrogen over methane of at least about 10, for hydrogen over propane of at least about 50, and for hydrogen over n-butane of at least about 100. Frequently, the hydrogen/methane selectivity achieved is 20 or more.
It is an object of the present invention to provide a membrane-based process for separation of hydrogen from a gaseous hydrocarbon.
Additional objects and advantages of the invention will be apparent from the description below to those of ordinary skill in the art.
It is to be understood that the above summary and the following detailed description are intended to explain and illustrate the invention without restricting its scope.